The invention relates to a method and apparatus for using an atomic absorption spectrometer to determine the content of volatile and decomposable compounds.
In the micro-determination of arsenic according to the method of flameless atomic absorption spectroscopy, it is known to transform the arsenic as contained in the sample material into inorganic arsenic (III) compounds and to react the same in such a way that arsine containing hydrogen is formed (see the paper of R. C. Chu, G. P. Barron, P. A. W. Baumgarner in Analyt, Chem. 44, 1476; 1972). The reaction vessel on the one hand is connected to an argon source and to a heatable cell of Vycor-glass on the other hand. The arsine containing hydrogen is, then, replaced by argon and thus flows through said heated measuring cell. At the elevated temperatures of the measuring cell, the arsine decomposes and forms arsenic, the atomic absorption of which may be measured under the conditions of the prevailing temperature.
It is also known (Bulletin M-2029; Beckman Instruments GmbH) that elements like antimony, bismuth, selenium, tellurium and others may be determined in similar manner.
The aforementioned methods and the apparatus for carrying out the same employ a flow of inert gas to transfer the gas mixture including the volatile, decomposable compound into the measuring cell of the atomic absorption spectrometer. This is advantageous because of the particularly small absorption by the inert gas itself; it is, however, of disadvantage that the proportion of the substance to be measured in the flowing gas is further reduced. The danger exists, therefore, that the compound contained in the gas flow will not be completely pyrolyzed so that it is reacted and measured only in part. Since the elements to be determined are predominantly of a kind which are present only in trace amounts, an incomplete decomposition of the volatile compound will unnecessarily impair sensitivity of the measurements.
A further disadvantage of the known apparatus is that the measuring cell is made of Vycor-glass or quartz which cannot withstand the temperatures required in flameless atomic absorption spectroscopy.
Accordingly, the principal objective of the invention is to provide a method and apparatus for carrying out the same in which heat is favorably transferred to the gas flow so that the volatile compounds become completely decomposed so that the atoms of the compound are introduced into the measurement chamber of an atomic absorption spectrometer without loss.
In accordance with the invention, this objective is achieved by the thermal decomposition occurring within a conduit prior to the entry into the measuring cell. This is achieved by heating the conduit to the decomposition temperature in a region before its junction with the measuring cell. In the region of the junction of the conduit and the measuring cell, both the conduit and the cell are heated to atomization temperature.
By such a separation of thermal decomposition and atomization, both processes can be conducted each under optimum conditions. This is accomplished since the volatile compounds have sufficiently high volatility that they are transferred with the inert gas flow into the measuring cell at the high temperatures of the conduit. If the measuring cell is heated to decomposition temperatures as well, complete decomposition of the gaseous test sample is assured.
In a modification of the method according to the invention, the conduit at a point adjacent the sample introduction orifice into the cell is heated to a temperature above the decomposition temperature while the measuring cell is not heated. Once the gaseous test sample is formed, the gaseous sample is introduced via the introduction orifice into the measuring cell and the measuring cell is, then, heated to the atomization temperature.
According to this modification of the invention, thermal decomposition and spectroscopic measurement of atomic absorption are separated in space and time. The end of the conduit adjacent the measuring cell is heated to a high temperature so that the decomposition products are deposited within the unheated measuring cell. If the temperature of the end of the conduit adjacent the measuring cell is chosen high enough, no deposit will form. The measuring cell can then be heated separately to a temperature favorable for the spectroscopic measurement of atomic absorption. Such a modification is particularly useful with atomic absorption devices having a graphite tube employed as measuring cell since the graphite tube has a sample introduction orifice in its wall through which said end of the conduit may readily become introduced into the graphite tube. Thus, installation of a specific quartz cuvette and the optical adjusting operations in the atomic absorption spectrometer associated therewith is unnecessary for the investigation of sample material containing elements which form volatile and thermally decomposable compounds.
In accordance with the invention there is provided for in the apparatus for carrying out the method, the conduit adjacent its opening into the measuring cell is furnished with a heater winding for heating the contents thereof at least to the decomposition temperature of the respective gaseous test sample.
According to the invention, the conduit may also have adjacent first and second heater windings at the conduit end adjacent the opening into the measuring cell. The second heater winding is located closest to the measuring cell and the windings may be separately energized. The first winding is generally used to heat the conduit to the decomposition temperature while the second heater may be used for heating the conduit to temperatures above the decomposition temperature.
By the configuration of two separately energizable heater windings disposed along the conduit adjacent the opening into the measuring cell, the processes of deposition and the process of measurement are separated in space and time. Advantageously, the first heater winding for thermal decomposition of the gaseous test sample is positioned upstream with respect to the second heater winding which causes some of the products of thermal decomposition to become deposited in the region of the second heater winding. By also heating the measuring cell to the decomposition temperature, decomposition products are deposited there as well. When the second heater winding and the measuring cell are simultaneously heated to the atomizing temperature, the decomposition products are atomized and carried by the inert gas into the measuring cell.
Suitably the conduit will be connected in a gas tight manner but detachably to the measuring cell. The conduit may open centrally into the measuring cell and said measuring cell may be made from quartz.
In a modification of the apparatus according to the invention, a temperature resistant metering tip is introduced into the sample introduction orifice of the measuring cell. The temperature resistant metering tip which may be made of sintered corundum is heated by energizing a winding therein to a temperatures in the range of 1000.degree. C. thereby atomizing the decomposition products in the conduit as they enter the measuring cell.
The conduit design described above is suitable for use with a graphite tube measuring cell since it may be inserted into the graphite tube. The design eliminates the need as typified by the prior art for a separate cell for the generation, decomposition and atomization of volatile, thermally decomposable compounds. Moreover, the usual arrangement of an atomic absorption spectrometer may be employed in the measurements to be presently described with just a slight widening of the customary sample introduction orifice in the wall of the graphite tube. This is advantageous because all adjusting operations connected with the introduction of another cuvette into the atomic absorption spectrometer are eliminated.
In accordance with the invention, the temperature resistant metering tip is locatable by a positioning means at either a first position where the metering tip projects through the sample introduction orifice into the interior of the measuring cell or at a second position where the metering tip is removed from the sample introduction orifice of the measuring cell.
By such positioning means, the temperature resistant metering tip can be positioned to project into the measuring cell only during the process of thermal decomposition. The metering tip may then be removed from the measuring cell after the formation of the gaseous test sample and its decomposition, said measuring cell being then available for the measurement of the atomic absorption of the decomposition products contained therein. Such an arrangement is particularly favored for carrying our the method of the invention because the decomposition products are formed immediately in the measuring cell, thus resulting in a decomposition product concentration which is high enough for accurate measurement by atomic absorption.
Advantageously, heating of the conduit, the metering tip and the measuring cell are adapted to be controlled in a preprogrammed manner by a control unit associated with the atomic absorption spectrometer. Accordingly, control of the resistant metering tip temperature, operation of the positioning means and heating of the measuring cell are adapted for preprogrammed control in such a way that said metering tip may be heated only in its first position and said measuring cell being heated only after removal of said metering tip. Thus the entire device for generating and decomposing the volatile compounds of the respectively desired element may be formed as an accessory component of a conventional atomic absorption spectrometer, the operation of said accessory component being incorporated into the controlled run of the operational program of said atomic absorption spectrometer.